Vertical cavity surface-emitting lasers (to be referred to as VCSEL hereinafter) are a known type of surface-emitting laser.
Such a surface-emitting laser has a pair of reflecting mirrors sandwiching an active region at the opposite sides thereof to form a resonator in a direction perpendicular to the substrate thereof and emits light in the direction perpendicular to the substrate.
The transverse mode output of such a surface-emitting laser is required to be a single mode output in view of applications thereof including electronic photography and telecommunications and hence developing a technique for controlling transverse mode of oscillation is an important problem.
For this reason, efforts are being paid to realize a single transverse mode in a surface-emitting laser by arranging a current confinement structure by way of selective oxidation in the inside of the device and thereby limit the light emission region of an active layer.
However, when realizing a single transverse mode only by arranging a current confinement structure, the confinement diameter needs to be reduced. As the confinement diameter is reduced, the light emission region is reduced to make it difficult to produce a large laser output.
In view of the above-identified problem, methods that can realize oscillations in a single transverse mode, while maintaining a relatively broad light emission region if compared with an instance of realizing a single transverse mode only by way of a current confinement structure, by intentionally introducing a loss difference between the fundamental transverse mode and higher order transverse mode, have been discussed.
As one of such methods, PTL 1 proposes a surface-emitting laser in which the reflectance distribution of a peripheral region surrounding the center region of the light emission region that corresponds to a light emission center region is made lower than that of the center region by forming a stepped structure by means of a semiconductor layer.
According to the PTL 1, the reflectance distribution of the peripheral region surrounding the center region becomes relatively lower than that of the center region by forming such a structure so that higher order transverse mode oscillations can be suppressed without reducing the optical output of the fundamental transverse mode.
A stepped structure as described above is referred to as surface relief structure hereinafter in this specification.